The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. Infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects will dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range corresponding to the particular detection function of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared energy in one of several ways. Thermal detectors respond to infrared energy by absorbing that energy and thus causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change the infrared radiation can be derived.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors), absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which leads to a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is affected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of focusing lenses constructed of materials transparent to infrared frequency energy, advances in semiconductor materials and the development of highly sensitive electronic circuitry have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, in a focal plane array, the outputs of which are connected to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitation, the processing circuitry can identify and monitor sources of infrared radiation.
Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared focal plane arrays have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
A contemporary subarray of detectors in a focal plane array may, for example, contain 256 detectors on a side, or a total of 65,536 detectors, the size of each square detector being approximately 88.9 mm (0.0035 inches) on the side with 12.7 mm (0.0005 inches) spacing between detectors. Such a subarray would therefore be 2.64 cm (1.024 inches on a side. Thus, interconnection of such a subarray to processing circuitry requires a connective module with sufficient circuitry to connect each of the 65,536 detectors to processing circuitry within a square, a little more than 2.54 cm (one inch) on a side. Subarrays may, in turn, be joined to form a large focal plane array that connects to 25 million detectors or more. Considerable difficulties are presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment.
The outputs of the detectors must undergo a series of processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression. By providing a signal processing module that performs at least a portion of the processing functions within the module, i.e. on integrated circuit chips disposed adjacent the detector focal plane, the signal from each detector need be transmitted only a short distance before processing. As a consequence of such on focal plane or up front signal processing, reductions in size, power and cost of the main processor may be achieved. Moreover, up front signal processing helps alleviate performance, reliability and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to the main signal processing network.
It is often desirable to construct a focal plane array sensitive to infrared radiation at a single or plurality of different frequencies. This may be done by disposing one or more optical filters in front of the detector elements. To provide a focal plane array having a plurality of frequency responses, the filter is typically divided into sections, i.e. stripes, each section transmitting a desired frequency band to dedicated detector elements. The focal plane arrays primary mirror may be toggled such that substantially the same image is alternately formed upon the different groups of detector elements located beneath the various filter sections or stripes. Each group of detector elements is thus responsive to one of the desired frequencies, i.e. disposed behind the appropriate optical filter section or stripe. Therefore, it is often desirable to mount filters in the optical path of the infrared detectors.
Prior art filters are commonly approximately 61.5 cm (2 feet) or more in diameter and approximately 0.64 cm (0.25 inch) thick. They must be large enough in diameter to cover the focal plane array and must be thick enough to avoid deformation due to mechanical vibration and thermal stress.
In order to obtain the desired frequency response, the stripes of the filter must be aligned such that infrared radiation passing therethrough falls upon the desired detector elements.
To minimize scattering and crosstalk, the filter must be located in close proximity to the detectors such that essentially all of the infrared radiation transmitted through a given portion of the filter is incident upon its associated detector element.
As such, although the prior art has recognized to a limited extent the problem of filtering infrared radiation prior to detection by focal plane arrays, the solutions have to date been ineffective in providing a satisfactory remedy.